The determination of motor vehicle speed is important to ensure motorist compliance with posted speed limits. Compliance with posted speed limits is recognized as critical for public safety. In general, vehicle speed detection has been based on radar gun systems. Radar guns typically use the emission and detection of reflected electromagnetic waves from the object being interrogated. The relative velocity of the object can then be determined by the Doppler effect on the reflected waves due to the vehicle's motion. The Doppler effect shifts the frequency of the radar reflection based on the relative velocity (speed) of the “target”, allowing for the direct and generally accurate measurement of speed of a moving vehicle.
However, there are some known problems with radar guns. First, for speed to be accurately calculated, the moving object of interest should be the only moving object in the beam path of the radar gun. If this is not the case, it is impossible to determine which object's speed the system is reading. Second, the incident angle of the beam path of the radar gun to the object can also affect the measurement. This includes angles on a horizontal and vertical planes. Third and perhaps most importantly, the active electromagetic emissions required in such systems, even in the case of laser emitting systems, can be detected by commerically available radar detectors procured by motorists purposefully trying to evade speed limit enforcement efforts.
Some have suggested use of a passive system one or more cameras, without an electromagnetic wave emission source, to determine the velocity of a moving object. However, the use of a camera images generally requires interpretation of the motion of a 3-dimensional (3D) object through 2-dimensional (2D) images provided by the camera. However, a primary problem with such an approach is that the 3D information of the vehicle is projected or nonlinearly transformed into 2D information. Hence, techniques or methods are needed to obtain 3D information despite the fact that only 2D information is available.
In one suggested method, as disclosed in “Identification of a moving object's velocity with a fixed camera”, by Chitrakaran et al., Automatica 41 (2005) 553-562, if a geometric length of a moving object is known, camera images from a single camera can be used to estimate the object's velocity, provided the initial position and orientation of the object is known a priori. However, because this method requires knowledge or an accurate estimate of a geometric length of the object, as well as the initial position and orientation of the object, this method does not permit images from a single camera to be used to determine an unknown or a random object's velocity with sufficient accuracy to be useful for vehicle speed detection applications.
Another disclosed passive system for measuring vehicle speeds is based on a system that requires image data from two cameras. U.S. Pat. No. 6,675,121, to Hardin et al., discloses a passive electro-optical range and total velocity measuring system having first and second cameras positioned along a common baseline. Control systems activate the first and second cameras at a first instance to capture a target image of a target at location T1 and at a second instance to capture a target image of the target at location T2. A range computer calculates ranges from the first camera, the second camera, and a baseline midpoint to a target at location T1 and location T2. An angles computer calculates target displacement. A velocity computer calculates total target velocity, track velocity, and cross-track velocity.
Although the Hardin system provides the advantage of being an undetectable speed measuring system as compared to convention emission based radar systems, the Hardin system is not readily mobile and requires two separate cameras for operation. Therefore, what is needed is a passive single camera speed detection system that is preferably adapted for mobility.